Fabric touch sensor

ABSTRACT

A planar (two-dimensional, XY location) touch sensor may include a knitted structure and supplementary method of sensing detects human touch on a fabric surface. This sensor may be fully knitted and detect the continuous planar location and contact force of human touch along the surface of the structure. The fabric may conform to any arbitrary surface and may be a rectangle for touch pad applications. This sensor may be used for applications that include robotics and human-machine interaction, smart garments and wearables, as well as medical textiles and flexible embedded sensors. This touch sensor may require as few as only two electrode connections from the fabric to sense both planar touch and pressure, which allows it to work in areas with limited space that allow for limited complexity for wiring.

BACKGROUND

Several groups have begun conducting research on the production of softflexible touch sensors. Recently, Google's Project Jaquard developed amethod of using industry standard jacquard machinery to produce textileswith integrated sensors for use in bespoke smart garments. GeorgiaTech's Healthcare Robotics Lab developed silicone sensors with“taxels”—tactile pixels—used to characterize force applied to a roboticarm. Other methods have also been investigated such as use of conductiverubber, layering of piezo-resistive and conductive textiles,combinations of conductive knit or woven textiles and threads, screenprinting, splicing of optical sensors into individual fibers andknitting of structures containing silver coated nylon, stainless steel,and carbon or polymeric conductive yarns. Sensors knit with carbon fiberfilament show may improve the sensitivity of wearable devices. Theknitting process helps with consistency of the design, while the carbonfiber has been shown to function well in a wider range of conditions.

While progress has been made, many of these solutions still face anumber of challenges with respect to manufacturability and robustness.Hard and fragile embedded electronic components and the need for bundlesof wire leads often diminish the feasibility of some solutions. Humanfactors can change the efficacy of these devices, for instance, the needto recalibrate antenna components that can function at differentfrequencies on the human body than in free space. Particularly in thecase of sewn sensors, the production process is lengthy, complex, andcannot easily conform to exact measurements. Additionally, the need towash and clean these garments or medical devices with sensors willarise, adding complexity to the design and production.

Previous fabric-based touch sensing has required a large number ofsensing electrodes (wires) to form a discrete sensing mesh or has used adense weaving of conductive yarn in an XY grid pattern to sense humantouch using self-capacitance or mutual capacitance. Covering a surfacewith discrete electrodes is impractical for scalability of the sensor asit increases the number of required connections to a sensing integratedcircuit.

SUMMARY OF THE EMBODIMENTS

A planar (two-dimensional, XY location) touch sensor may include aknitted structure and supplementary method of sensing detects humantouch on a fabric surface. This sensor may be fully knitted and detectthe continuous planar location and contact force of human touch alongthe surface of the structure. The fabric may conform to any arbitrarysurface and may be a rectangle for touch pad applications. This sensormay be used for applications that include robotics and human-machineinteraction, smart garments and wearables, as well as medical textilesand flexible embedded sensors. This touch sensor may require as few asonly two electrode connections from the fabric to sense both planartouch and pressure, which allows it to work in areas with limited spacethat allow for limited complexity for wiring.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B shows a sensor structure comprising three layers ofknitted fabric: a nonconductive layer, a spacer fabric layer, and aconductive sensing element layer.

FIG. 2 shows a cross section of the conductive carbon fiber yarn.

FIG. 2A shows a resistive strain gauge.

FIG. 3A shows the sensor structure in use.

FIG. 3B shows an example of input signals given a square wave input.

FIG. 4 shows the resistor circuit.

FIGS. 5A and 5B show touch force and contact area with the sensor.

FIG. 6 shows the path of the sensing element.

FIG. 7 shows the coordinates in the weft and warp directions as afunction of x.

FIGS. 8A-D show alternative sensor designs.

FIGS. 9A and 9B show a sensor embodiment with buttons.

FIGS. 10A and 10B show a button touchpad and corresponding circuit.

FIG. 11 shows a node current diagram.

FIG. 12 shows the Equation 11.

FIGS. 13A-C show Equations 12a-c.

FIGS. 14-18 show summary mathematical reductions of the mathematicsperformed to determine certain data from the sensor.

FIGS. 19A and 19B show a series RC circuit with its plotted output.

FIG. 20 shows a series of steps in creating conducting elements usingthreads.

FIGS. 21A and 21B show an interlock knit stitch pattern and spacer knitstitch pattern.

FIG. 22 shows a fabric circuit sensing diagram.

FIG. 23 shows a fabric sensor GUI.

DETAILED DESCRIPTION OF THE EMBODIMENTS Introduction

A planar (two-dimensional, XY location) touch sensor may include aknitted structure and supplementary method of sensing detects humantouch on a fabric surface. This sensor may be fully knitted and detectthe continuous planar location and contact force of human touch alongthe surface of the structure. The fabric may conform to any arbitrarysurface and may be a rectangle for touch pad applications. This sensormay be used for applications that include robotics and human-machineinteraction, smart garments and wearables, as well as medical textilesand flexible embedded sensors.

This touch sensor may require as few as only two electrode connectionsfrom the fabric to sense both planar touch and pressure, which allows itto work in areas with limited space that allow for limited complexityfor wiring. The sensor may be scaled to fit large and arbitrary surfacesand material efficiency due to needing only two wire connections. Theknitted structure may include a single piece using industry standardflatbed knitting techniques. Furthermore, the knitted structure mayrequire no embedded electronic or solid components to be placed in thefabric which allows the sensor to be flexible and resilient.

Touch-sensitive interfaces offer unique and robust levels of interactionbetween users and touch-enabled devices. In the last 10 years, humancomputer interaction (HCI) reaped tremendous benefit from the design anddevelopment of such interfaces, now ubiquitous in smartphones andtablets. Other HCI areas, such as robotics and wearable technology,could benefit from sensors that detect touch, especially if they couldbe made soft and flexible. The number of touch sensing applications ispoised to increase as research and development of soft touch sensors ispursued—specifically in the development of smart textiles for wearableapplications and soft robotics.

A soft wearable interface could aid in patient-physician interaction,helping to remotely and comfortably monitor health conditions. As asmart garment component, soft sensors could be used as a means ofentering information into a device or to provide kinesthetic feedback tothe wearer. These sensors could even act as controls to “alter”properties of the garment when combined with other smart materials thatcan modulate color or temperature. In the field of robotics, soft touchsensors could improve the quality of human-robot interaction. While lowcost depth cameras have revolutionized aspects of human-robotinteraction in terms of environment mapping and kinematic planning,tactile sensing, which directly correlates to the robot's dynamics, isoften neglected. This is due in part to rigid construction and controlschemes which cannot account for unplanned collisions. Soft sensorswould alleviate some of these issues by allowing deformation at contactpoints which may well relax tighter kinematic constraints and furtherenhance the ability for robots to operate in a wide range ofenvironments.

Knitting may be one method of producing soft, flexible sensors thataddresses some of the challenges mentioned above. Knitting is a methodof fabric production that has existed for thousands of years, and hasbeen successfully mechanized over the past several hundred years. Weftknitting is the intermeshing of horizontal rows of loops to create afabric that can stretch in both the horizontal and vertical directions.When automated, the process becomes a type of additive manufacturing andrapid prototyping similar to 3D printing as it creates a substrate byadding layers on top of layers. However, digital knitting provides anumber of advantages over the more recently developed 3D printingtechnologies. Software suites used with Shima Seiki industrial knittingmachines can virtually drape garments and simulate the knitting processto evaluate designs before production. Additionally, a wide range ofmaterials in the form of yarn have already been tested and establishedfor use in knitting machines. Furthermore, garments can be knit withmultiple materials seamlessly within the same piece of textile. Theprocess creates little waste and, if only a small quantity of a materialis available for testing, it can still be incorporated in smallsegments. Seamless knitting also eliminates potential points ofstructural weakness and provides a platform for the development of softcircuitry. When combined with techniques such as knitted spacer fabrics,knitting can be used to create 3D forms that are suitable for electronicdevices while remaining soft, flexible, and comfortable for use ingarments.

Designers with knowledge of materials engineering envision functionaland aesthetically pleasing products while taking into considerationmaterial capabilities. Materials scientists and engineers with knowledgeof human factors work to design, produce, and characterize new materialsor utilize and provide an understanding of existing materials that willfulfill the functional and aesthetic needs the final product and provideinsight into the end-user's interaction. Mechanical and electricalengineers with knowledge of design assist with characterization oftextile structure and assess their physical and electrical propertiesand make changes that feed back into the designers' original concept.Together, these groups work at the intersection between theirs andothers' disciplines to incorporate both design and engineering intocreating a textile-based sensor.

Sensor Structure

FIGS. 1A and 1B show a sensor structure 100 comprising three layers ofknitted fabric: a nonconductive layer 110, a spacer fabric layer 120,and a conductive sensing element layer 130. The nonconductive layer 110may be a standard cotton, wool, or synthetic yarn knitting that provideselectrical isolation between the rows 132 of the sensing elementpattern. The nonconductive layer 110 acts as the sensor 100's backing toprevent unwanted electrical contact with the sensing element layer 130.The spacer fabric layer 120 comprises a knitted high-bulk nylon thatprovides a tactile rigidity for the sensing element 100 and resistslongitudinal deformation of the sensing element layer 130. Thenonconductive layer 110 and the sensing element layer 130 may completelyenclose the spacer fabric layer 120, as best shown in FIG. 1B.

A conductive yarn 200 may form the patterned sensing element 130. Theconductive yarn 200 may be a multi-strand twisted monofilament carbonfiber yarn shown in more detail in FIG. 2. The carbon fiber yarn 200 isa continuous knitted strip traversing the face of the sensor 130alternating in the horizontal (weft) direction from the base to the topof the rectangular structure in a pattern similar to that of a resistivestrain gauge 250 shown in FIG. 2A. Additionally, the structure mayinclude a heat melt yarn 140 at a top seam that is steam-sealed toprevent the fabric from unraveling. Electrodes (not shown) may attach tothe endpoints 134, 136 of the carbon fiber layer 130 at the yarn entryand exit outputs 150, 152.

Yarn Structure

The sensing element 130 may include a spun stranded monofilament carbonfiber yarn 200 comprised of multiple strands 210 wound together. Eachsuccessive yarn wrapping may be counter-twisted (S and Z twisted) tobalance the yarn 200, for example 2S-4Z-4S shown in FIG. 2. As shown,pairs of strands 210 twist clockwise to form dual strands 220. 4 dualstrands may be counterclockwise-twisted to form an octet strand 230. Andfour octet strands 230 form may be twisted clockwise to form a singlefiber yarn 200. Different strand combinations are of course possible.

The resistivity of a single strand of carbon fiber filament may beapproximately 1 MΩper inch. The yarn impedance is determined by thenumber of strands twisted into the yarn. This number is calculated as afunction of the size of the sensing area, the desired stitch pattern,and the stitch tightness. The impedance of the sensing element ismatched to the impedance of the current limiting resistors used in thecircuit to attain the best signal results. The average capacitanceinduced by touch is typically between 70 to 120 pico-Farads (pF). Theaverage observed parasitic capacitance is typically between 6 to 20 pF.Resistor values close to 1 Mega-Ohm (MΩ) are ideal for both the currentlimiting resistors and the total resistance of the sensing element. Thisvalue yields a measurable signal rise time without increasing thevoltage fluctuations induced from parasitic capacitance.

Single-Touch Sensing Method

FIG. 3B and FIG. 4 show the sensor 100 measures a human touch location330 by means of self-capacitance. Human skin introduces a capacitiveconnection between the sensing element 100 and the electrical potentialof the human body, which alters the shape of a reference signal. Thesensor 100 may interface with an external microcontroller (such as anAtmel SAM3X8E) and filter circuit. The microcontroller may generate a500 Hz square wave input with a 50% duty cycle to both first inputs 310,320 of the conductive sensing element, though the frequency is variable.The pulses are timed to charge and discharge synchronously. Capacitanceis measured by examining the voltage change between first and secondcurrent limiting resistors 312, 322 and the sensing element 130. Thesecurrent limiting resistors 312, 322 and first and second exit outputs150, 152 are separated by first and second nodes 313, 323. Theresistor-capacitor (RC) rise time is found through measuring the timeneeded to charge/discharge the circuit to 90%/10% of the operatingvoltage. FIG. 3B shows an example waveform output based on touchposition.

In use, the sensor 100 first uses a self-calibration routine to measureboth the baseline parasitic capacitance and unknown total impedance.This routine is important to account for the uncertainties in theresistor matching as well as changes in impedance due to deformation ofthe sensor. The sensor 100 self-calibrates by setting the voltage highand low at either input of the sensor and measuring the voltage at bothoutput junctions. The impedance ratio nay be measured and used inconjunction with the known current limiting impedance to determine thetotal impedance of the sensing element. When the initial RC voltagecurve is measured, the baseline parasitic capacitance can be determinedas a function of the known total impedance and measured RC rise time.

The purpose of sensing a return signal from both ends of the fabricenables the sensor 100 to account for both the unknown total impedanceand induced capacitance but also allows the sensor to discriminatebetween the position of the touch input and the touch force by providingtwo independent variables as output. The touch position and touch forceare coupled as a function of the signal rise times—the touch positionitself is coupled as a function of the linear length of the sensingelement. A method of measuring binary force (touch/no touch) involvessimple thresholding when either rise time crosses a certainpredetermined value. Linear distance is calculated as the differencebetween the values of the two rise times. Linear distance x may beapproximated from the rise times t_(rA) and t_(rB) byx≈log(t_(A))/log(t_(rB))

Calculation of Unknown Resistances

The sensing element 100 contains three resistive elements, RA, R_B, andR_C. Resistances R_A and R_B are the contributions of the currentlimiting resistors (312, 322 in FIG. 3) and R_C 412 (FIG. 4) representsthe total resistance of the sensing element 100.

The normalized resistances are shown in equations (1a) and (1b).

$\begin{matrix}{{\frac{R_{i}}{R_{A} + R_{B} + R_{C}} = {\hat{R}}_{i}},{i = A},B,C} & {{Equation}\mspace{14mu}\left( {1a} \right)} \\{{{\hat{R}}_{A} + {\hat{R}}_{B} + {\hat{R}}_{C}} = 1} & {{Equation}\mspace{14mu}\left( {1b} \right)}\end{matrix}$

When measuring coarse single touch input, leakage current out of thesystem, I_(A) _(p) and I_(B) _(p) , is assumed to be less than the mainbranch currents, I_(A) and I_(B), due to a high filter input impedance.I _(A) _(p) <<I _(A) ,I _(B) _(p) <<I _(B)

During calibration, voltage at alternate ends of the sensor is set highand low to direct current towards either output terminal and generateuniform current flow. The normalized voltage at either input terminal ismeasured and used to determine the unknown normalized resistancesthrough formulas (2a) and (2b).{circumflex over (V)} _(A) I={circumflex over (R)} _(A)  Equation (2a)(1−{circumflex over (V)} _(B))I={circumflex over (R)} _(B)  Equation(2b)

Detecting Touch Position

The transient response of the output signal given a step input ofmagnitude V_(CC) yields a first order LTI system equation in (3).

$\begin{matrix}{{{V(t)} = {V_{CC}\left( {1 - e^{- \frac{t}{RC}}} \right)}},{t \geq 0}} & {{Equation}\mspace{14mu}(3)}\end{matrix}$

Where R and C are the respective equivalent resistance and capacitanceof the circuit. Rise time, t_(r), is defined as the time needed to reach90% of the final, normalized signal value as shown in equation (4).

$\begin{matrix}{0.9 = {1 - e^{- \frac{t_{r}}{RC}}}} & {{Equation}\mspace{14mu}(4)}\end{matrix}$

The electrical potential at the touch contact point is equivalent forboth branches of the circuit, A and B. Because the output voltage atleads A and B 150, 152 is output in parallel, the responses can beequated as in equation (5a). The sensing element acts as a linearvoltage divider when touch is input. The touch position, x, is definedas the normalized distance from lead A.

$\begin{matrix}{{1 - e^{- \frac{t_{r_{A}}}{{({R_{A} + {R_{C}x}})}C}}} = {1 - e^{- \frac{t_{r_{B}}}{{({R_{B} + {R_{C}{({1 - x})}}})}C}}}} & {{Equation}\mspace{14mu}\left( {5a} \right)}\end{matrix}$

Equation (5a) is simplified to yield equation (5b). Notice that theinduced capacitance drops out of the equation in (5c).(R _(B) +R _(C)(1−x))Ct _(r) _(A) =(R _(A) +R _(C) x)Ct _(r) _(B)  Equation (5b)R _(B) t _(r) _(A) −R _(A) t _(r) _(B) +R _(C) t _(r) _(A) =(R _(C) t_(r) _(B) +R _(C) t _(r) _(A) )x  Equation (5c)(R _(B) +R _(C))t _(r) _(A) −R _(A) t _(r) _(B) =R _(C)(t _(r) _(B) +t_(r) _(A) )x  Equation (5d)

The equation in (5d) is rearranged to solve for x in (5e).

$\begin{matrix}{x = \frac{{\left( {R_{B} + R_{C}} \right)t_{r_{A}}} - {R_{A}t_{r_{B}}}}{R_{C}\left( {t_{r_{B}} + t_{r_{A}}} \right)}} & {{Equation}\mspace{14mu}\left( {5e} \right)}\end{matrix}$

The normalized resistances are proportional to the actual resistancethrough equation (1a). Both the numerator and denominator are divided bythe total circuit resistance to yield equation (5f).

$\begin{matrix}{x = \frac{{\left( {{\hat{R}}_{B} + {\hat{R}}_{C}} \right)t_{r_{A}}} - {{\hat{R}}_{A}t_{r_{B}}}}{{\hat{R}}_{C}\left( {t_{r_{B}} + t_{r_{A}}} \right)}} & {{Equation}\mspace{14mu}\left( {5f} \right)}\end{matrix}$

Detecting Touch Force

Capacitance may be represented as a function of touch contact areaC=f(A), where C is capacitance and A is area, which is itself a functionof contact force, A=f(F), where F is force. As shown in FIGS. 5A, and5B, when a finger 510 makes contact with the sensing element 130 insidea contact area 520. Depending on the force applied in the touch, a lighttouch finger application 510 a creates a smaller area than a heaviertouch finger application 510 b to create a larger contact area 520 b.the skin deforms and depending on the touch force applied, the contactarea increases. Touch force detection is important to discriminatebetween gestures like pressing, which generally has a heavy touch, andswiping, which is done with a lighter touch. The measure of capacitance,however, is influenced by the parasitic capacitance of the circuit.Parasitic capacitance is present in the sensing leads and throughout thesensing circuit. During calibration, the sensor measures the baselinerise time of the circuit as shown in Equation (6). The value t_(r) _(p)represents the rise time as influenced by the parasitic capacitance.This value forms the baseline rise time used to distinguish touch andnon-touch.

$\begin{matrix}{0.9 = {1 - e^{- \frac{t_{r_{p}}}{{RC}_{p}}}}} & {{Equation}\mspace{14mu}(6)}\end{matrix}$

Binary touch can be detected by means of thresholding. A threshold risetime, t_(r) _(threshold) , is chosen as a value greater than thebaseline rise time but less than the rise time of a light touch. A touchevent occurs when either measured rise time is greater than thethreshold rise time. Conversely, no touch is registered while both risetimes are below the threshold rise time. The piecewise function is shownin Equation (7).

$\begin{matrix}{{f\left( {t_{r_{A}},t_{r_{B}}} \right)} = \left\{ \begin{matrix}0 & {{t_{r_{A}}\mspace{14mu}{and}\mspace{14mu} t_{r_{B}}} < t_{r_{threshold}}} \\1 & {{t_{r_{A}}\mspace{14mu}{or}\mspace{14mu} t_{r_{B}}} \geq t_{r_{threshold}}}\end{matrix} \right.} & {{Equation}\mspace{14mu}(7)}\end{matrix}$

An approximation of the continuous touch force is in Equation (8). Theforce is approximated by averaging the rise times of signals A and B andsubtracting the baseline rise time.

$\begin{matrix}{p = {\frac{t_{r_{A}} + t_{r_{B}}}{2} - t_{r_{p}}}} & {{Equation}\mspace{14mu}(8)}\end{matrix}$

Position Mapping

Mapping linear position to a 2D plane may be accomplished by means of alook up table or by analytically mapping the linear touch position via aparametric equation. Using a look up table may be useful when the sensorarea is non-uniform or when there are changes in resistance thatseparate the linear resistance into piecewise functions. It may also beuseful when discretizing the output position, such as in the case of abutton array. A parametric equation is useful when continuity of outputis desired, such as in the case of a track pad.

FIG. 6 shows the path of the sensing element. The normalized lineardistance, x, acts as the independent variable of the parametricequations that define the touch location coordinates. FIG. 7 shows thecoordinates in the weft and warp directions as a function of x. * * *Can you provide some more context and explanation for these figures?

Software and Drivers

The software may be used from the libraries included in the Arduino IDEand the Atmel Software Framework (ASF).

The software to sense touch may run embedded on the microcontroller usedto interface with the knitted fabric sensor. The microcontroller maystream data over a serial port to a connected PC. Ancillary software anddrivers are currently being developed to visualize touch input but isnot necessary to the functionality of the sensor.

The software may enable application for the sensor, but software choicesmay vary.

Sensor Design

A planar (two-dimensional, XY location) touch sensor with multi-touchand multi pressure sensing capabilities includes a knitted structure andsupplementary method of sensing that detects human touch on a fabricsurface. This sensor may be fully knitted and may detect the continuousplanar location and contact force of human touch along the surface ofthe structure. In addition, the sensor may be knitted to conform to anyarbitrary surface but is commonly knitted as a rectangle for touch padapplications. This sensor may have certain applications including thosediscussed above and including robotics and human-machine interaction,smart garments and wearables, as well as medical textiles and flexibleembedded sensors.

Sensor Design

The sensor designs shown in FIGS. 8A-D show possible planar knittedshapes with the sensing element and although these designs are shown inthe context of this discussion about a multi-touch sensor, some of thedesign elements are applicable to the other sensing elements. Theplacement of the sensing element in all designs is non-intersecting. Thefirst shape in FIG. 8A represents a touch pad 800 including a sensor 830with buttons 835 that are connected continuously between both inputs850, 852. The buttons 835 may be knitted into the top layer of thestructure and are not tactile—they detect change in capacitance causedby applied pressure. The buttons 835 may be formed by knitting theconductive yarn above or below the surface of the top layer discussedearlier with respect to FIG. 1.

The second shape in FIG. 8B shows a sensor 830 b an arbitrary 2D planarshape with an arbitrary, non-intersecting sensing element path. Thethird and fourth shapes in FIGS. 8C and 8D show knitted pattern sensors830 c, 830 d for capacitive sliders in the form of a linear slider andwheel respectively. All patterns may be able to detect multiple touchpoints, each with distinct touch pressures.

As shown in FIGS. 9A and 9B, which show alternate embodiments of thetouch pad 800 in FIG. 8A, the touch pad 900 includes buttons 935 and asoutputs, conductive snaps 950, 952 (the sensor path in FIG. 9A is shownon a white background to make it more visible). The carbon fiber yarnsensor may be knitted in a continuous path traversing the face of thesensor alternating in the horizontal (weft) direction from the base tothe top (warp) of the rectangular structure as the path of the carbonfiber trace. Additionally, the structure may include a heat melt yarn atthe top seam that is steam-sealed to prevent the fabric from unraveling.Electrodes may connect to the conductive snaps 950, 952 at electrodeoutputs A and B. FIG. 8B shows the path of the sensor 930 through thetouch pad 900.

Sensing

The sensing method uses projected self-capacitance to measure both thelocation and pressure of human touch. FIG. 10A shows a user's hand 1090pressing multiple touch points 1020 with each finger 1010 applying adistinct capacitance 1012 a, 1012 b, etc. The circuit 1000 shown in FIG.10B is charged from a step response or square wave input at points V_(A)1080 and V_(B) 1082. The resulting charge is absorbed through acombination of the touch points 1020 and the parasitic capacitance ofthe circuit 1000. In multi-touch and multi pressure sensing, the shapeof the output waveform is analyzed to determine the pole-zero placementof the system which correlates to the touch capacitance and touchlocation.

The flow of current at each touch point is depicted in the node diagramin FIG. 11. Current flow out of the node represents a positive current.Each node has a total current summation of zero as defined in Kirchoff'sCurrent Law.

Calculations Using Circuit

Using modified nodal analysis (MNA) and the circuit diagrams in FIGS.10B and 11, the mathematical representation of the circuit's electricalbehavior may be found. For each node, the current moving towards a lowervoltage potential out of the node is considered positive. The currentbetween the nodes of a resistor is found in Equation 9a while thecurrent between the nodes of a capacitor is found in Equation 9b. In thecase of the second equation, the tem s represents the Laplace variable.The conductivity, G, is equal to the inverse of the resistance R.

$\begin{matrix}{{i = {\frac{\Delta\; V}{R} = {G\left( {\Delta\; V} \right)}}},{G = R^{- 1}}} & \left( {{Equation}\mspace{14mu} 9a} \right) \\{i = {{C\left( {\frac{d}{dt}{\Delta V}} \right)} = {C\left( {{\Delta V}\; s} \right)}}} & \left( {{Equation}\mspace{14mu} 9b} \right)\end{matrix}$

The total current exiting a touch point node can be found using Equation10.

$\begin{matrix}{{\frac{e_{j} - e_{j - 1}}{R_{K_{j - 1}}} + \frac{e_{j} - e_{j + 1}}{R_{K_{j}}} + {C_{t_{j}}e_{j}s}} = {{{G_{K_{j - 1}}\left( {e_{j} - e_{j - 1}} \right)} + {G_{K_{j}}\left( {e_{j} - e_{j + 1}} \right)} + {C_{t_{j}}e_{j}s}} = 0}} & \left( {{Equation}\mspace{14mu} 10} \right)\end{matrix}$

The general circuit diagram can be written in matrix form as shown inEquation 11, FIG. 12.

To solve for the output voltages, o_(A) (s) and o_(B)(s), use Cramer'sRule (Equations 12a, 12b, and 12c) to substitute the solution vectorinto the columns of the matrix whose elements we wish to solve foro_(A)(s) as shown in Equations 12a, 12b, and 12c in FIGS. 13A-C. Theoutput in both cases yields strictly-proper transfer functions—the orderof which is n+2, where n is the number of touch points. The variables ofthe equation are as follows:

G_(A):  Known; Measurable G_(B):  Known; MeasurableG_(K):  Known; Measurable G_(K₀)  …  G_(K_(n − 1)):  Unknown${G_{K_{n}}\text{:}\mspace{14mu}{Calculable}\mspace{14mu}{from}\mspace{14mu} G_{K}} - {\sum\limits_{n - 1}^{i = 0}G_{K_{i}}}$C_(t₁)  …  C_(t_(n)):  Unknown C_(P_(A)):  Unknown C_(P_(B)):  Unknown

Determining the Knitted Resistance

The first three unknown terms, R_(A), R_(B), and R_(K), are measured byapplying a positive voltage to one input while grounding the secondinput. The measured voltage across outputs A and B yield the ratio ofthe resistances.

FIG. 14 summarizes the No Touch scenario mathematics that yields thesecond order system V_(o) _(A) (s) which is summarized in FIG. 15.

Because of the near pole-zero cancellation, the dominant pole may be areal-axis pole and because the transient response is primarilyfirst-order, the dominant pole is closer to the right-hand side of theplot. Thus, the term beneath the radical is positive and the resultantof the operation is positive.

Time domain response:v _(A)(t)=V _(CC)(1−e ^(λt))  (Equation 15)with 5 measured terms and 5 degrees of freedom may be further expandedas shown in FIG. 16.

Single Touch

The single touch mathematics may be seen as follows.

$\begin{matrix}{{\begin{bmatrix}G_{A} & {- G_{A}} & 0 & 0 & 0 & 1 & 0 \\{- G_{A}} & \left( {G_{A} + G_{K_{0}} + {sC}_{P_{A}}} \right) & {- G_{K_{0}}} & 0 & 0 & 0 & 0 \\0 & {- G_{K_{0}}} & \left( {G_{K_{0}} + G_{K_{1}} + {sC}_{t_{1}}} \right) & {- G_{K_{1}}} & 0 & 0 & 0 \\0 & 0 & {- G_{K_{1}}} & \left( {G_{K_{1}} + G_{B} + {sC}_{P_{B}}} \right) & {- G_{B}} & 0 & 0 \\0 & 0 & 0 & {- G_{B}} & G_{B} & 0 & 1 \\1 & 0 & 0 & 0 & 0 & 0 & 0 \\0 & 0 & 0 & 0 & 1 & 0 & 0\end{bmatrix}\begin{bmatrix}v_{A} \\o_{A} \\e_{1} \\e_{2} \\e_{3} \\o_{B} \\v_{B} \\i_{V_{A}} \\i_{V_{B}}\end{bmatrix}} = \begin{bmatrix}0 \\0 \\0 \\0 \\0 \\0 \\0 \\V_{A} \\V_{B}\end{bmatrix}} & \left( {{Equation}\mspace{14mu} 16} \right) \\{{V_{o_{A}}(s)} = {{G_{A}G_{B}G_{K_{0}}V_{A}} + {G_{A}G_{B}G_{K_{1}}V_{A}} + {G_{A}G_{K_{0}}G_{K_{1}}V_{A}} + {G_{B}G_{K_{0}}G_{K_{1}}V_{B}} + {C_{P_{B}}C_{t_{1}}G_{A}V_{A}s^{2}} + {C_{P_{B}}G_{A}G_{K_{0}}V_{A}s} + {C_{P_{B}}G_{A}G_{K_{1}}V_{A}s} + {C_{t_{1}}G_{A}G_{B}V_{A}s} + {C_{t_{1}}G_{A}G_{K_{1}}V_{A}s}}} & \left( {{Equation}\mspace{14mu} 17} \right) \\{{s^{0}\text{:}\mspace{14mu} G_{A}G_{B}G_{K_{0}}} + {G_{A}G_{B}G_{K_{1}}} + {G_{A}G_{K_{0}}G_{K_{1}}} + {G_{B}G_{K_{0}}G_{K_{1}}}} & \left( {{Equation}\mspace{14mu} 18} \right) \\{{s^{1}\text{:}\mspace{14mu} C_{P_{B}}G_{A}G_{K_{0}}} + {C_{P_{B}}G_{A}G_{K_{1}}} + {C_{t_{1}}G_{A}G_{B}} + {C_{t_{1}}G_{A}G_{K_{1}}}} & \left( {{Equation}\mspace{14mu} 19} \right) \\{s^{2}\text{:}\mspace{14mu} C_{P_{B}}C_{t_{1}}G_{A}} & \left( {{Equation}\mspace{14mu} 20} \right) \\{\Delta = {{G_{A}G_{B}G_{K_{0}}} + {G_{A}G_{B}G_{K_{1}}} + {G_{A}G_{K_{0}}G_{K_{1}}} + {G_{B}G_{K_{0}}G_{K_{1}}} + {C_{P_{B}}G_{A}G_{K_{0}}s} + {C_{P_{B}}G_{A}G_{K_{1}}s} + {C_{P_{A}}G_{B}G_{K_{0}}s} + {C_{P_{A}}G_{B}G_{K_{1}}s} + {C_{P_{A}}G_{K_{0}}G_{K_{1}}s} + {C_{P_{B}}G_{K_{0}}G_{K_{1}}s} + {C_{t_{1}}G_{A}G_{B}s} + {C_{t_{1}}G_{A}G_{K_{1}}s} + {C_{t_{1}}G_{B}G_{K_{0}}s} + {C_{t_{1}}G_{K_{0}}G_{K_{1}}s} + {C_{P_{A}}C_{P_{B}}C_{t_{1}}s^{3}} + {C_{P_{A}}C_{P_{B}}G_{K_{0}}s^{2}} + {C_{P_{A}}C_{P_{B}}G_{K_{1}}s^{2}} + {C_{P_{B}}C_{t_{1}}G_{A}s^{2}} + {C_{P_{A}}C_{t_{1}}G_{B}s^{2}} + {C_{P_{A}}C_{t_{1}}G_{K_{1}}s^{2}} + {C_{P_{B}}C_{t_{1}}G_{K_{0}}s^{2}}}} & \left( {{Equation}\mspace{14mu} 21} \right) \\{a_{0} = {C_{P_{A}}C_{P_{B}}C_{t_{1}}}} & \left( {{Equation}\mspace{14mu} 22} \right) \\{a_{1} = {{C_{P_{A}}C_{P_{B}}G_{K_{0}}} + {C_{P_{A}}C_{P_{B}}G_{K_{1}}} + {C_{P_{B}}C_{t_{1}}G_{A}} + {C_{P_{A}}C_{t_{1}}G_{B}} + {C_{P_{A}}C_{t_{1}}G_{K_{1}}} + {C_{P_{B}}C_{t_{1}}G_{K_{0}}}}} & \left( {{Equation}\mspace{14mu} 23} \right) \\{a_{2} = {{C_{P_{B}}G_{A}G_{K_{0}}} + {C_{P_{B}}G_{A}G_{K_{1}}} + {C_{P_{A}}G_{B}G_{K_{0}}} + {C_{P_{A}}G_{B}G_{K_{1}}} + {C_{P_{A}}G_{K_{0}}G_{K_{1}}} + {C_{P_{B}}G_{K_{0}}G_{K_{1}}} + {C_{t_{1}}G_{A}G_{B}} + {C_{t_{1}}G_{A}G_{K_{1}}} + {C_{t_{1}}G_{B}G_{K_{0}}} + {C_{t_{1}}G_{K_{0}}G_{K_{1}}}}} & \left( {{Equation}\mspace{14mu} 24} \right) \\{a_{3} = {{G_{A}G_{B}G_{K_{0}}} + {G_{A}G_{B}G_{K_{1}}} + {G_{A}G_{K_{0}}G_{K_{1}}} + {G_{B}G_{K_{0}}G_{K_{1}}}}} & \left( {{Equation}\mspace{14mu} 25} \right) \\{\frac{a_{1}}{a_{0}} = {\frac{G_{A} + G_{K_{0}}}{C_{P_{B}}} + \frac{G_{B} + G_{K_{0}}}{C_{P_{A}}}}} & \left( {{Equation}\mspace{14mu} 26} \right) \\{\frac{a_{2}}{a_{0}} = \frac{{G_{A}G_{B}} + {G_{A}G_{K_{0}}} + {G_{B}G_{K_{0}}}}{C_{P_{A}}C_{P_{B}}}} & \left( {{Equation}\mspace{14mu} 27} \right)\end{matrix}$

Controllable Canonical Form

$\begin{matrix}{A = \begin{bmatrix}1 & 0 & 0 \\0 & 1 & 0\end{bmatrix}} & \left( {{Equation}\mspace{14mu} 28} \right)\end{matrix}$

Pole Locations

$\begin{matrix}{\lambda_{A} = \begin{bmatrix}\; \\\;\end{bmatrix}} & \left( {{Equation}\mspace{14mu} 29} \right)\end{matrix}$

Two or More Touch Points

The output voltage signals from a multi-touch input exhibit a similartransient response as a single-touch input or no touch in that thehigher-frequency poles are paired close to corresponding zeros and theunpaired pole is slower-moving and dominates the behavior of thetransient response, that mathematics for which is summarized in FIGS. 17and 18.

Example Applications

An example application of the sensor involves its use as a touch sensorfor a humanoid robot. The sensor swatches are placed on the arms of therobot and are used to detect human touch. Touching different locationsof the sensor controls the movement of the arms to move towards or awayfrom the location of touch.

Another example application of the sensor involves its use as a trackpad to move a cursor on a computer screen. The sensor measures planarlocation on a rectangular swatch and converts the position into themovement of the cursor.

How Capacitive Sensing Works

Projected-capacitive sensors are among the most commonly used touchsensors in computing and mobile devices. A capacitive sensor is ameasurement device that converts a measured change in capacitance into acontinuous or discrete output. In the case of detecting human touch, acapacitive sensor may measure the induced capacitance of the human bodythrough the change in the dielectric coefficient to detect whether ornot a touch has occurred. A basic capacitive sensor uses a resistor andcapacitor in series to form a circuit 1900 as shown in FIG. 19A, and itscorresponding time vs Vout graph FIG. 19B. The input is driven by asimple on-off voltage generator, V_(in), and the output is a change inthe voltage waveform across the capacitor, V_(out). The circuit 1900 ismodeled as a first-order linear ordinary differential equation. Thecapacitor is sensitive to changes in charge and the resistor issensitive to changes in current (derivative of voltage). Furthermore,because only one branch current is present in the circuit 1900, only asingle measured point (time-voltage pair) is needed to determine thevalue of the capacitance given a known input voltage and resistance. Asthe values of both resistance and capacitance increase, the timeconstant, τ, increases. τ is defined as the resistance multiplied by thecapacitance of the circuit. In the case of measuring the capacitance ofhuman touch, the induced capacitance is small—in the range of 10's ofPico-Farads. Thus, a large series resistance is needed to measure therise time while using a modest sampling interval.

Theory of Materials/Yarns

In the sensor described herein, the touch sensor structure combines withresistive and non-resistive yarns to create an alternating grid-likepattern. The main body of the knit structure, which is non-resistive,may be made from two ends of Primaloft® yarn, (50% Primaloft, 50% wool,3.5 twists per inch) and the sensing element may be made from a filamentcarbon fiber yarn with a linear resistance of approximately 1 MΩ/in.Carbon fiber may be chosen as the sensing element material because ofits high resistivity. Furthermore, the linear resistance of the yarn maybe tailored to match the desired total resistance of approximately 1 MΩby twisting multiple filaments together. The carbon fiber yarn that maybe used in the sensor may be made from a commercially available carbonfiber monofilament (Resistat, Type F901, Merge 5022, 22 Denier, 24 Dtexfrom Shakespeare Conductive Fibers). To produce a yarn with the desiredresistance, 32 ends of carbon fiber monofilament may be twisted togetherusing a Simet Twisting Machine following the steps illustrated in FIG.20, and summarized earlier with reference to FIG. 2.

In FIG. 20, two ends of carbon filament were wound together onto fourindividual cones, 2010. The four cones from step one were S-twistedtogether and wound onto four new cones, 2020. The four cones from steptwo were then Z-twisted together onto one cone, 2030.

Theory of Knit Structures

While resistance can be changed by using different types of resistiveyarns, other techniques to alter the resistance of the sensor involvechanging the knit architecture. This can mean increasing the number ofcourses and wales—by increasing the length of the courses (thehorizontal dimension of the knit) which increases the resistance or byincreasing the wales (the vertical dimension of the knit) whichdecreases the resistance.

Architecture can also mean changing the area of the pattern and contactarrangement of the yarn by creating an interlock patterns. Interlockpatterns 2100 create thicker courses by drawing more yarn across theneedle bed (FIG. 21A). The pattern misses (floats) stitches between knitstitches to deposit more material over multiple passes. A technique toincrease the thickness of the pattern itself involves the use of “spaceryarn” that tucks between the front and back needle beds to fill in thegap (FIG. 21B) and creates a hybrid pattern 2110. The spacer yarn “bulksup” the fabric to create a spongy texture, an architecture for touchsensors.

Theory of Sensing and Operation

As described previously, capacitive sensing may be measured throughchanges in voltage. The sensing circuit 2200 depicted in FIG. 22 modelsthe touch pad as having a continuous linear resistance, R_(K). When thesensor is touched at a point, a pathway to ground is created. Thecontinuous resistance is then split into two resistances, R_(K)x andR_(K)(1−x) that are proportional to the normalized touch location, x.

The sensor detects the linear touch location by measuring the rise andfall times of the voltage outputs, V_(out), at either end of the knittedfabric sensor. A square wave pulse is generated at the sources, V_(in),and passed through current limiting resistors, R_(A) and R_(B). Thevalues of R_(A), R_(B), and R_(K) should match as closely as possible toprovide the best range of output. Inexact matching of resistors R_(A)and R_(B) will cause skewed voltage readings and the reported touchlocation will be biased towards the higher value resistor.

The voltage sensing is performed by an external microcontroller (AtmelSAM3X8E). T The microcontroller may generate a 500 Hz square wave inputwith a 50% duty cycle to both input leads of the current limitingresistors, though the frequency may vary. The pulses are timed to chargeand discharge synchronously. Capacitance and position are measured byrecording the time needed to charge the circuit to ½ of themicrocontroller's output voltage. These times range from 10 to 70microseconds depending on the touch pressure and relative charge of theindividual. Touch interactions induce oscillations in the outputwaveform and skew the measured rise time. Filtering is performed on therise time data through a simple moving average. The operation steps arelisted in Table 1.

Step Fabric circuit sensing procedure. 1 The circuit is discharged; theinput signal is pulsed HIGH. 2 The system time is recorded as thestarting time of the rising signal 3 Hardware interrupts trigger foreach output terminal that measures when the signal crosses the lowthreshold. 4 The interrupts return the system time at the instant uponreaching ½ rise time as the ending time. 5 The microcontroller returnsthe difference of the starting and ending times. 6 The circuit ischarged; the input signal is pulsed LOW. 7 The system time is recordedas the starting time of the falling signal. 8 Hardware interruptstrigger for each output terminal that measures when the signal crossesthe high threshold. 9 The interrupts return the system time at theinstant upon reaching ½ fall time as the ending time. 10 Themicrocontroller returns the difference of the starting and ending times.

Modeling and Simulation

In order to verify the observed circuit behavior, the circuit and touchinteractions were modeled using MATLAB Simulink and Simscape ElectricalFoundation Library. A relationship was sought that decouples the touchlocation and capacitance given two output rise times. This relationshipis useful for creating a capacitance-invariant touch position model tosense touch location from different users, each with their own baselinecharge.

A model of the physical circuit and microcontroller functions wassimulated over a range of touch positions and capacitances spanning from1 to 200 pico-Farads to determine a model that decouples the touchlocation and touch pressure from the output rise times. Though a 30 to60 Hz oscillation was present in waveforms observed from the physicalcircuit, no attempt was made to replicate this noise in the simulatedmodel.

Graphical User Interface

A graphical user interface (GUI) 2300 was created to indicate theregistered touch location and pressure to provide visual feedback duringtesting. The GUI indicates the touch location on the vertical black barsby means of an indicator. The program uses a simplistic algorithm todetermine the touch location by taking the difference of the A and Belectrode readings and dividing by the sum of the readings. This valueindicates the offset from the center of the pad. For instance ifreadings A and B are equal, the output value will be close to the centerof the pad. If reading A is much greater than reading B, the value willskew towards the position of electrode A and vice-versa. The sensitivitymeter displays the raw readings from the electrodes along with themaximum readings from each to assess imbalances in the sensing circuit.

Results and Discussion

To verify the modeled data, the simulated output was compared againstreal world data. Data was collected from six individuals who were askedto press on all 36 discrete sensor pads. 100 data samples were taken perindividual per pad, amounting to 600 data points per pad for 36 pads. Toconvert the pad locations to a real numbered position, the data waslabeled with the normalized distance between the two endpoints, rangingfrom 0 to 1 in divisions of 35. Aside from a simple moving averageapplied to the data within the microcontroller, no additional filteringwas applied. Furthermore, only the position information was recordedduring testing. The touch capacitance was not measured. This was due inpart to the inability to accurately measure capacitance in theexperimental setup but also to verify the hypothesis that the measuredtouch location would be invariant to the touch capacitance. Nocalibration procedure was performed on the sensor in between testing tosimulate the effect of real world use.

While each individual exhibited a different base charge per dataset, theoverall spread of the data matched the expected output provided by thesimulation. Data from individuals who had a higher touch capacitanceshowed a more pronounced spread between discrete touch points. Thismatches the predicted distribution of the position as touch capacitanceincreases. Furthermore, the separation of data between the left, middle,and right regions of the pad is distinct and indicates that coarse touchlocation can be accurately performed. Quantitatively, the data had aroot-mean-square error (RMSE) value between the expected and observedvalues of 0.225. This error physically correlates to a misclassificationof the normalized distance of approximately one-fourth of the length ofthe sensing element. The discrepancy between the model data and the datacollected is likely a result of differences in the model's assumedresistance versus the actual resistance and from the induced noise.

While the invention has been described with reference to the embodimentsabove, a person of ordinary skill in the art would understand thatvarious changes or modifications may be made thereto without departingfrom the scope of the claims.

The invention claimed is:
 1. A touch sensor structure comprising: aconductive sensing element layer comprising a conductive yarn and havingtwo exit outputs, wherein the conductive yarn comprises a multi-strandtwisted monofilament carbon fiber yarn; a nonconductive layer thatprovides a backing for the conductive sensing element layer and preventsunwanted electrical contact with the sensing element layer; and a spacerfabric layer that resists longitudinal deformation of the conductivesensing element layer; wherein upon a capacitive touch on the sensinglayer, the touch sensor structure can determine a position of the touch;wherein the conductive sensing element layer comprises the two exitoutputs; wherein the two exit outputs comprise a first exit output and asecond exit output, wherein the first exit output is connected to afirst input and a second exit output is connected to a second input;wherein a microcontroller generates a wave input with a 50% duty cycleto each of the first input and second input, wherein the wave inputpulses are timed to charge and discharge synchronously; wherein a firstcurrent limiting resistor is located between and directly connected tothe first input and the first exit output, and the first exit output islocated between and directly connected to the first current limitingresistor and the sensing element layer; and a second current limitingresistor is located between and directly connected to the second inputand the second exit output, and the second exit output is locatedbetween and directly connected to the second current limiting resistorand the conductive sensing element layer.
 2. The touch sensor structureof claim 1, wherein the nonconductive layer and the sensing elementlayer enclose the spacer fabric layer.
 3. The touch sensor structure ofclaim 1, wherein the conductive sensing element layer comprises acontinuous strip that traverses a face of the touch sensor structure. 4.The sensor structure of claim 3, wherein the continuous strip shapeincludes parallel rows of the sensing element layer.
 5. The sensorstructure of claim 4, wherein the rows are in a weft direction withinthe touch sensor structure.
 6. The touch sensor structure of claim 1,wherein the twisting of the microfilament fiber yard occurs in twodirections.
 7. The touch sensor structure of claim 1, further comprisinga heat melt yarn at a top seam of the touch sensor structure that issteam-sealed to prevent the touch sensor structure from unraveling. 8.The touch sensor structure of claim 1, wherein the exit outputs aresnaps.
 9. The touch sensor structure of claim 1, wherein the exitoutputs attach to electrodes.
 10. The touch sensor structure of claim 1wherein the wave input is a 500 Hz square wave input.
 11. The touchsensor structure of claim 1, wherein the capacitance of the capacitivetouch is measured by comparing the voltage change between first andsecond current limiting resistors and the sensing element layer.
 12. Thetouch sensor structure of claim 11, wherein the capacitive touch isintroduced into the sensing element layer through the electricalpotential of the human body introduced to the sensing element layerthrough a touch of the human body.
 13. The touch sensor structure ofclaim 12, wherein a distance from output A to the touch of the humanbody on the sensing element layer is given by the formula$x = \frac{{\left( {{\hat{R}}_{B} + {\hat{R}}_{C}} \right)t_{r_{A}}} - {{\hat{R}}_{A}t_{r_{B}}}}{{\hat{R}}_{C}\left( {t_{r_{B}} + t_{r_{A}}} \right)}$where x is the distance, RA, RB, and RC are the resistances of the firstresistor, second resistor, and sensing element layer resistancerespectively, and trA and trB are the rise times from the first andsecond inputs respectively.
 14. The touch sensor structure of claim 1,wherein the sensing element layer further comprises buttons.
 15. Thetouch sensor structure of claim 1, wherein the sensing element layeralso measures the force applied.
 16. The touch sensor of claim 15,wherein the force applied is measured by measuring the capacitance ofthe capacitive touch.
 17. The touch sensor of claim 1, wherein the firstoutput and the second output are connected to electrodes.